BERKELEY, CA — Type Ia supernovae are among the best
standard candles known to astronomy — objects whose distance can
be determined because their intrinsic brightness is known or can be computed,
just as the distance to a 100-watt bulb can be calculated by comparing
its apparent brightness with its actual brightness.

Determining the expansion rate of the universe by comparing the brightness
and redshift of far-off Type Ia supernovae therefore critically depends
on accurate measurements of both.

Looking through dust
in galaxies can make supernovae appear dimmer, but dust also makes
their light redder — just as dust in our atmosphere makes
sunsets appear red. (Galaxy image: 2MASS, with simulated SN)

One worrisome possible source of error in measuring distant supernovae
has been host-galaxy extinction, the filtering effect of dust peculiar
to the galaxy in which the supernova occurs. Dust occurs in our own galaxy
too, but has been so extensively studied that it is of less concern in
supernova distance measurements.

The concern is that distant supernovae appear dimmer not because of the
accelerating effects of dark energy but, more prosaically, because of
dust. There is a straightforward way to distinguish these effects, however,
since dust normally reddens the light passing through it. Shorter, bluer
wavelengths are absorbed and scattered more readily than longer, redder
wavelengths.

"When you want to measure a supernova's brightness you can measure
the light that was blue when it left, or the light that was red,"
says Greg Aldering, a member of the Supernova Cosmology Project and leader
of the Nearby Supernova Factory program, which concentrates on studying
the intrinsic properties of Type Ia supernovae. "Both measurements
are valid, but what you want is to make sure you get the same answer on
both sides of the spectrum. If the blue is fainter, you've got a dust
problem."

Imagine supernovae
as stoplights. Does the green light appear fainter because it's
farther away (top) or because it has been absorbed and scattered
by dust (bottom)? We can tell by comparing the brightness of the
red light: if dust made the green light fainter, the red light will
not be fainter by as much.

The extraordinarily high quality of photometric data from the 11 distant
supernovae studied by the Hubble Space Telescope in this study allowed
their intrinsic brightness to be determined and compared in both bands.

The study determined that no anomalous effects of host-galaxy extinction
occur at great distance; distant supernovae are comparable to nearby supernovae
in this respect, underlining their utility as standard candles.